What happened to my FITS structural keywords?
The CXC Datamodel supports multiple data formats including FITS and various ASCII formats. It provides a view of the file to the user and to the applications in a file-neutral format. This means that many of the format specific, structural keywords that describe the columns, units, descriptions, WCS, etc, are all normally hidden from the user. For example if we look for the TTYPE keywords that describe the FITS columns they are not shown with a normal dmlist command.
unix% dmlist evt.fits header | grep TTYPE (nothing returned)
Adding the raw option will display all the structural keywords.
unix% dmlist evt.fits header,raw | grep TTYPE Key 11: C *TTYPE1 = time / S/C TT corresponding to mid-exposure Key 14: C *TTYPE2 = ccd_id / CCD reporting event Key 18: C *TTYPE3 = node_id / CCD serial readout amplifier node Key 22: C *TTYPE4 = expno / Exposure number of CCD frame containing event Key 25: C *TTYPE5 = chipx / Chip coords Key 28: C *TTYPE6 = chipy / Chip coords Key 35: C *TTYPE7 = tdetx / ACIS tiled detector coordinates Key 38: C *TTYPE8 = tdety / ACIS tiled detector coordinates Key 45: C *TTYPE9 = detx / ACIS detector coordinates Key 48: C *TTYPE10 = dety / ACIS detector coordinates Key 55: C *TTYPE11 = x / sky coordinates Key 58: C *TTYPE12 = y / sky coordinates Key 65: C *TTYPE13 = pha / total pulse height of event Key 71: C *TTYPE14 = pha_ro / total read-out pulse height of event Key 77: C *TTYPE15 = energy / nominal energy of event (eV) Key 82: C *TTYPE16 = pi / pulse invariant energy of event Key 88: C *TTYPE17 = fltgrade / event grade, flight system Key 92: C *TTYPE18 = grade / binned event grade Key 96: C *TTYPE19 = status / event status bits
The values displayed are processed through the same rendering mechanism as all other keywords so the raw values may have different format and/or precision then compared to what is in the physical file.
dmhedit may be able modify some of the reserved keywords, however, users are strongly discouraged from using it in this way. There are usually better ways in CIAO to modify such information without directly editing any reserved keywords.